If you do not necessarily need a zoom lens than also have a look at the 300mm f4 IS which also works great as a 420mm f5.6 IS lens with the 1.4x TC and offers stabilization unlike the 400mm f5.6.
Also, if you insist on a zoom, then the two Sigmas are great and excellent value for money (had the first one and now I have the 2nd one for my wife): Sigma 120-400mm OS, Sigma 150-500mm OS.
Finally, if you can stretch your budget a bit, the new Sigma 50-500mm OS is supposed to be excellent.
All Sigma lenses above have image stabilization that is supposed to be better than the one in the Canon models above (300mm and 100-400) as it offers 1-2 extra stops (more likely one stop from my tests).